1. Technical Field
The present invention relates to channel fading in a mobile telecommunications network and, more particularly, efficient power control between a transmitter and a receiver of a radio signal undergoing fading.
2. Discussion of Related Art
A base station or radio network controller will normally be in radio communication with several mobile stations within its coverage area. It is desirable for uplink signals from the mobile stations to the base station to be received with the same signal power. Because the mobile stations are at different distances from the base station they cannot transmit using fixed power levels since mobile stations close to the base station would have too strong a signal and mobile stations far away would be too weak. This is called the near-far effect and is solved by uplink power control. The faraway mobile station transmits with more power than the mobile station close to the base station so they are received with the same signal power.
Usually, there are two types of power control used, i.e., open-loop and closed-loop. The open-loop method requires the mobile station to measure channel interference and adjust its own transmission power according to the measured interference without any interaction with the base station. This approach has certain problems, particularly where the uplink and the downlink are transmitted on different frequencies and fading on the different frequency carriers are not particularly well correlated.
Fading exists because a single line-of-sight radio path may not exist from the mobile station to the base station. Consequently, the base station may observe the superposition of many copies of the same signal from the mobile station, each one with a different delay. These delayed signals are due to the reflection from objects such as trees, cars, buildings, etc. If the mobile station is moving, the copies of the signal can add constructively or destructively and the signal received at the base station appears to fluctuate, or fade wildly.
In an uplink closed-loop power control approach, the quality of the uplink from the mobile station to the base station is measured in the base station. Power control commands formulated in the base station are adjusted accordingly and sent back to the mobile station so that it adjusts its transmission power correspondingly. This gives faster and better results than the above-mentioned open-loop approach, but it still cannot react to rapid fading as well as might be desired. A known way to address this problem is to make the closed-loop measurements and reporting very fast. However, the transmit power control (TPC) commands that are sent e.g., every time slot (667 microseconds) are still only commanding a small increment of change in power while the fading may be more extreme. In that case it takes a long time for the power control commands to elicit the desired response and by that time the fading may be changing in the opposite direction.
An object of the present invention is to provide an enhanced power control between a radio transmitter and receiver to counteract fast signal fading variations in the channel.
According to a first aspect of the present invention, a method comprises the steps of determining that a radio uplink channel power fluctuation exists, and changing a spreading factor used for uplink channel spreading to counteract said power fluctuation.
Further according to the first aspect of the invention, the change in spreading factor is an increase due to the channel power fluctuation determined as comprising a decreasing power fluctuation.
Further still according to the first aspect of the invention, the change in spreading factor is a decrease due to the channel power fluctuation determined as comprising an increasing power fluctuation.
Still further according to the first aspect of the invention, the step of determining is carried out in a radio network and wherein the method further comprises the step of sending a spreading factor control signal from the network to a mobile station followed by the mobile station carrying out the step of changing the spreading factor.
Yet still further according to the first aspect of the invention, the step of changing is carried out by the mobile station only if the mobile station independently determines that the step of changing the spreading factor is permissible.
Further in accord with the first aspect of the invention, the method farther comprises the step of sending a transmit power control (TPC) signal from the network to the mobile station and wherein the step of determining comprises the step of counting instances of the TPC signal at the network and carrying out the step of determining by determining at the network if a count of the instances exceeds a selected number.
Yet further in accord with the first aspect of the invention, the selected number is adaptable.
Further still in accord with the first aspect of the invention, the selected number is adaptable within a window comprising a selected plurality of TPC signal instances.
Further according to the first aspect of the invention, the step of determining further comprises the step of determining a frame or block error rate of the radio uplink channel and wherein the step of changing the spreading factor is carried out only if the frame or block error rate meets a selected criterion.
Further still according to the first aspect of the invention, the step of determining comprises the step of detecting in a radio network the radio uplink channel power fluctuation in the radio uplink channel from a mobile station to the network, and wherein the method further comprises the step of transmitting a transmit power control (TPC) signal from the network to the mobile station commanding a change in transmit power to counteract power fluctuation.
Yet further still according to the first aspect of the invention, the step of determining comprises the step of counting instances of said TPC signal at the network or at the mobile station and carrying out the step of determining by determining at the network or at the mobile station if a count of the instances exceeds a selected number followed by the mobile station carrying out the step of changing the transmit power in response to a command signal from the network or in response to the count at the mobile station exceeding the selected number.
Yet still further according to the first aspect of the invention, the step of changing is carried out by the mobile station only if the mobile station independently determines that the step of changing the spreading factor is permissible.
Yet further still according to the first aspect of the invention, step of counting, at the mobile station, instances of the TPC signal and wherein the step of determining comprises the step of determining at the mobile station if a count of the instances exceeds a selected number.
Further in accord with the first aspect of the invention, the step of determining is carried out in a mobile station.
Yet further in accord with the first aspect of the invention, the step of counting, at the mobile station, instances of a transmit power control (TPC) signal received from a radio network and the step of determining comprises the step of determining at the mobile station if a count of the instances exceeds a selected number.
According to a second aspect of the invention, an apparatus comprises means for determining that a radio uplink channel power fluctuation exists, and means for changing a spreading factor used for uplink channel spreading to counteract said power fluctuation.
Further according to the second aspect of the invention, the change in spreading factor is an increase due to said channel power fluctuation determined as comprising a decreasing power fluctuation.
Still further according to the second aspect of the invention, the change in spreading factor is a decrease due to the channel power fluctuation determined as comprising an increasing power fluctuation.
Further still according to the second aspect of the invention, the means for determining is located in a radio network and wherein said apparatus further comprises means for sending a spreading factor control signal from the network to a mobile station to means for deciding a change in spreading factor.
Yet further according to the second aspect of the invention, the change is carried out by the mobile station only if the means for deciding independently determines that the change in the spreading factor is permissible.
Yet still further according to the second aspect of the invention, the apparatus further means for sending a transmit power control (TPC) signal from the network to the mobile station and wherein the means for determining comprises means for counting instances of the TPC signal at the network and means for deciding at the network if a count of the instances exceeds a selected number.
Yet further still according to the second aspect of the invention, the selected number is adaptable.
Further in accord with the second aspect of the invention, the selected number is adaptable within a window comprising a selected plurality of TPC signal instances.
Further still in accord with the second aspect of the invention, the step of determining further comprises means for determining a frame or block error rate of the radio uplink channel and wherein the means for changing the spreading factor changes the spreading factor only if said frame or block error rate meets a selected criterion.
Yet still further in accord with the second aspect of the invention, the means for determining comprises means for detecting in a radio network the radio uplink channel power fluctuation in the radio uplink channel from a mobile station to the network, and the apparatus further comprises means for transmitting a transmit power control (TPC) signal from the network to the mobile station commanding a change in transmit power to counteract the power fluctuation.
Yet further still in accord with the second aspect of the invention, the means for determining comprises means for counting instances of the TPC signal at the network or at the mobile station and means for determining at the network or at the mobile station if a count of the instances exceeds a selected number followed by the mobile station changing the transmit power in response to a command signal from the network or in response to the count at the mobile station exceeding the selected number.
Further according to the second aspect of the invention, the means for determining further comprises means for determining a frame or block error rate of the radio uplink channel and wherein the means for changing the spreading factor changes the spreading factor only if the frame or block error rate meets a selected criterion.
Yet further according to the second aspect of the invention, the means for changing is carried out by the mobile station only if the mobile station independently determines that the changing the spreading factor is permissible.
Further still according to the second aspect of the invention, the means for counting, at the mobile station, instances of TPC signal and wherein the means for determining comprises means for determining at the mobile station if a count of the instances exceeds a selected number.
Still further according to the second aspect of the invention, the means for determining further comprises means for determining a frame or block error rate of the radio uplink channel and wherein the means for changing the spreading factor changes the spreading factor only if the frame or block error rate meets a selected criterion.
Yet further still according to the second aspect of the invention, the means for determining is located in a mobile station.
Yet still further according to the second aspect of the invention, the apparatus further comprises means for counting, at the mobile station, instances of a transmit power control (TPC) signal received from a radio network and the means for determining comprises means for determining at the mobile station if a count of the instances exceeds a selected number.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing.